The excitation of a plasma by high-frequency power current supplies is known, for example, from semiconductor coating, plasma etching, laser excitation, etc. For various reasons, it is necessary to measure and monitor the current supplied to a plasma system by the current supply.
There is known a coil-based system for measuring relatively high high-frequency currents of up to several tens of amperes with relatively few distortions caused by electrical and electromagnetic fields less than 100 V/m.
In new developments in plasma technology, current measurement in high-frequency plasma applications is required in which currents flow far in excess of 100 A at from one to several hundreds of MHz and voltages greater than 1000 V, often even greater than 5000 V, are present at the current conductors, at frequencies of from 0 (direct voltage) up to several hundreds of MHz. At a distance of a few cm (e.g., less than about 10 cm) from a current-carrying conductor which is at a potential of 1000 V, interference fields of 10 kV/m and above are produced and can negatively influence the current measurement or even make current measurement impossible with conventional measurement devices.
The currents which can be induced in the coil of a conventional current measurement arrangement would be in many instances too high to be reasonably discharged directly to a connection. In general, unacceptable losses and measurement distortions would be produced if a large current on the order of 1 A, with signals in the MHZ range (1-100 MHz) were to be conducted to a measurement signal evaluation unit by means of a measurement cable.
Honea et al., “Improved construction of Rogowski coils for measurement of plasma currents”, Journal of Physics E: Scientific Instruments 1974, Volume 7, pages 537, 538, disclose the use of a Rogowski coil for measuring currents. The Rogowski coil has a shield which is connected to the coil conductor so that the shield and the coil conductor are at the same potential in at least one location.
What is needed is a current measurement device with which high-frequency currents can be precisely measured even when strong interference fields are present.